A color filter employed for a color liquid crystal display device usually comprises red, green and blue pixels formed on a light-transmissive support (e.g., glass plate) and a black matrix provided on a space area (gap area) between the pixels to improve contrast of a displayed image. The color filter is, for example, prepared by forming a black matrix in the form of net and forming the red, green and blue pixels in order on the space area of the black matrix utilizing photolitho method.
The black matrix is, for example, formed by known methods of employing a layer of metal such as chromium, and utilizing a photosensitive resin containing shading pigments dispersed therein. The method employing the metal layer comprises the steps of forming a metal layer on a support by depositing, and forming a photoresist layer, exposing imagewise the layer to light and developing the exposed layer (i.e., photolitho method), and further etching an exposed area of the metal layer. The method using photosensitive resin containing a shading pigment comprises the steps of forming a photosensitive resin layer containing a shading pigment by means of coating or printing, pattern-wise exposing the layer to light and developing the exposed layer.
Japanese Patent Provisional Publication No. 62(1987)-9301 discloses a process for preparing a black matrix by a self-alignment exposing system (self-register system) using a photosensitive black resin containing a shading pigment dispersed therein. The self-alignment exposing system attracts considerable attention from the standpoint of workability and economical efficiency. The process comprises the steps of forming a photosensitive black resin layer containing a shading pigment (e.g., carbon black) dispersed therein on a support provided with red, green and blue pixels, and exposing to light the photosensitive black resin layer on the back side of the support so that the pixels serve as a photomask.
In more detail, the red, green and blue pixels are formed by coating a coating solution containing a polyimide precursor for forming each of the pixels on a support; the formed layer is exposed to light and developed to form color pattern (pixels); and on the color pattern, a layer of casein wherein photosensitivity is given by ammonium dichromate is formed by coating; and then the layer is exposed to light from the back side by using the color pattern as a photomask and developed, to prepare a black matrix. The self-alignment exposing system brings about an advantage of requiring no register step, which is usually needed for forming a black matrix, and further of scarcely producing a small image-lacking portion because the photosensitive casein layer for forming a black matrix is formed so as to completely cover the gap area (space area) between the pixels.
Japanese Patent Provisional Publication No. 7(1995)-28236 discloses an image forming method wherein transmittance of visible radiation and that of ultraviolet radiation of a photosensitive black resin layer containing a shading pigment are controlled in the range of 1:20 to 0:1, whereby a black matrix formed of the photosensitive black resin layer has a high optical density.
In the self-alignment exposing system, the photosensitive black resin layer containing a shading pigment, which has been exposed to light on the back side of the support, is not sufficiently cured, as described in the above Publication. In more detail, a black pigment (e.g., carbon black) used as a shading pigment greatly adsorbs light of the wavelength region where the photosensitive black resin layer shows photosensitivity, and therefore light incident upon the back side of the support does not reach to a upper side (surface side) of the photosensitive black resin layer not to give a sufficient curing condition in the vicinity of the surface. Hence, the exposed portion of the photosensitive black resin layer is locally removed in the vicinity of the upper surface, which results in lowering of the optical density due to decrease of a thickness of the photosensitive black resin layer.
The control of transmittance in a wavelength region where the photosensitive black resin layer shows photosensitivity, described in Japanese Patent Provisional Publication No. 7(1995)-28236 mentioned above, brings about improvement of the curing condition because of increase of transmittance of light for exposure.
However, it is difficult to prepare a black matrix having a high optical density (e.g., not less than 3.0).